This disclosure relates to green joining of ceramic components, and more particularly, is directed to a method of joining green ceramic components of the type used in a high intensity discharge (HID) lamp application. However, this application should not be limited to that particular application and may find application and benefit in related environments and uses.
A need exists for a simple and inexpensive manner of bonding ceramic components together. It is known by way of other disclosures, to mechanically assemble the components or parts, then place the assembled components into a debindering furnace for removal of the wax, a typical binder, so that exposure over a preselected time and temperature causes the components to bond. A portion of these components experience what appear to be hoop-stress cracks extending generally parallel to an axis of the assembled components. These cracks appear to develop during the debindering cycle in the furnace, and it is believed that the cracks are caused at least in part from an interference fit and the resulting forces that are transferred from one component to the other. Thus, although the known process is successful in mechanically bonding the components together, any cracks are an undesired limitation in forming a hermetic bond between the components that is critical to lamp functionality. Thus, a need exists to bond or secure the components before debindering so that cracks do not form in the finished part.
Other approaches have been proposed in an effort to join the green components before debindering. For example, U.S. Pat. No. 6,620,272 describes a process for joining green ceramic components through localized heating. Specifically, the '272 patent describes a ceramic arc tube used in HID lamps comprised of a polycrystalline alumina ceramic. An alumina powder is mixed with a binder material such as a wax or thermoplastic, formed into a desired shape, e.g., by injection molding, and the binder material assists the molded alumina components in maintaining a desired shape while in a green state. As is known, the binder is then removed when the components are fired. The resultant arc tube advantageously withstands high stress, temperature, and corrosive chemicals associated with the arc tube environment.
The '272 patent suggests that heat be applied to joining surfaces so that a localized melting occurs at that region. Exemplary methods of localized heating include heated gas or forced hot air, an infrared laser, an incandescent lamp, or an incandescent resistive element applied to the joining surfaces. In addition, after heating, the '272 patent then subjects one or both components to compression and stretching, where compression urges the components toward one another and past any initial point of contact. This compression results in a radially outward extending bulge being formed at a visible seam between the joined components. The patent description states that the bulge is then reduced by subsequent stretching. This compression and stretching may occur multiple times, and even though the completed arc tube allegedly achieves desired performance characteristics, a visible seam remains after sintering.
Excessive handling of bonded green components result in cracks and/or undesirably subject the joined component to forces and stresses that may lead to cracks in the final part. Thus, a need exists for a simple, inexpensive, quick-acting, commercially adaptable, and highly effective process for joining green ceramic components and particularly a process that reduces the cracks in the final parts, and preferably does not have a visible parting line or seam.